This invention relates to an electron beam exposure system, and more particularly to an electron beam exposure system of electron projection type adapted for use in processing on an industrial scale of ultra large-scale integrated circuits (ultra LSIs), storage or operation cells using magnetic bubble devices, microwave transistors, photo-integrated circuits, etc.
A method is known of making a microminiature pattern on devices such as ICs by printing such pattern on a substrate (hereinafter referred to as a wafer) coated with a photosensitive agent (hereinafter referred to as a photoresist). Heretobefore, light has been typically used to print the pattern on the substrate. Such optical method however is not applicable to, for example, processing ultra LSIs that require a line width of 1 micron or less. Exposure method has therefore been developed using X-ray or electron beams of short wavelengths. One example of this exposure method is in electron beam writing system that prints a pattern on a wafer by scanning in any order the wafer with a single very narrow beam of electrons (a beam diameter, for example, of about 0.1 micron) as in a scanning type electron microscope. While this method has advantages of providing adequate resolution and allowing computer controlled scanning with electron beams, it has the disadvantage of requiring prolonged exposure time that prevents large-scale processing. The exposure time that depends on the scanning method or the kind of photoresist is typically at least several tens of minutes for a wafer 5 to 7 cm square. Therefore, the method is only applicable to the manufacture of an experimental product or a mask (which will be described hereinafter) that serves as a model of a pattern to be printed. A recent modification called the "rectangular beaming system" that uses an electron beam whose cross section is shaped by a fixed or adjustable aperture stop has the exposure time per wafer reduced to only a few minutes. But then the modified method gives lines as wide as at least 2 microns; in other words, it cannot offer a reduction in exposure time without sacrificing the line width. Another method that uses X-rays offers an advantage of relatively simple and less expensive equipment, but on the other hand, with a line width requirement of less than 1 micron, the method is not suitable for large-scale processing not only because of difficulties in the manufacture and registration of the mask mentioned above but also it takes as much exposure time as the electron beam writing system.
The electron beam exposure system of electron projection type of which this invention is an improvement employs a wide beam of electrons and causes it to transmit through a pattern cut in a mask of metal foil located on the way of its path. The electron beam which has its cross section as obtained immediately after passage through the cut pattern shaped to comply with said pattern is projected on the wafer to form an electronic optical image of the pattern thereon. Consequently, the amount of electron beams that can be utilized is very large as compared with the electron beam exposure system according to the afore-described electron beam writing system that uses a very narrow electron beam; hence, the exposure time for a wafer 5 to 7 cm square is reduced to only several tens of seconds. For this reason, the electron beam exposure system of electron projection type is considered basically suitable for mass production of very small circuit elements.
However, according to this type of electron beam exposure system that has an intended pattern cut in a metal foil mask, that portion of the mask which is enclosed by the pattern of a closed loop cannot be held in position. One method of solving this problem is to retain the whole mask on the electron paths by placing it on a screen made of extremely fine wires; this eliminates the possibility of a dotted image on the wafer by blurring. However, blurring to prevent the dotted image in turn makes it difficult to form an accurate image of the pattern on the wafer; secondly, stable retaining of separate portions of the mask on the wire screen is not so easy as desired; and thirdly, the wire screen scatters the electron beam, thus reducing the resolution and accuracy.
Alternatively, such looped patterns cut in the mask can be divided into more than one sub-pattern so that no single sub-pattern has a closed loop. For exposure, a set of such fragmentary patterns are combined together to form on the wafer a complete pattern that includes the looped portions. However, according to the conventional method of such "overlapping exposure system", different masks are used for different sub-patterns, thus resulting in a very low productivity because a great care need be used in replacement or registration of respective masks.